Florescent lamp having electrodes comprising a tubular braid and an additional wire coiled about the same space



5, 1967 R A. MENELLY ETAL 3356883 FLUORESCENT LAMP HAVING ELECTRODES COMPRISING A TUBULAR BRAID AND AN ADDITIONAL WIRE COILED ABOUT THE SAME SPACE Filed Dec. 21, 1964 .d j 4 -INVENTORS ZZZ/20rd .27. Merle)? jeoaora (K271279724 i United States Patent 3 356,883 FLORESCENT LAMP H AViNG ELECTRODES COM- PRISING A TUBULAR BRAID AND AN ADDI- TIONAL WIRE COILED ABOUT THE SAME SPACE Richard A. Mcuelly, Danvers, and Theodore J. Pricenski, Ipswich, Mass., assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed Dec. 21, 1964, Ser. No. 419,946 4 Claims. (Cl. 313-211) ABSTRACT OF THE DISCLOSURE A fluorescent lamp with an emissive electrode supported by and connected between two lead wires, the electrode comprising a tubular braid of relatively high resistance, emissively coated wire, and an additional, relatively low resistance wire coiled about the same space as the braid and connected in parallel therewith.

Fluorescent lamps, for example, typically comprise an elongate glass envelope filled with mercury and an inert gas and having electrodes at each end of the lamp connected to outside terminals. A high voltage alternating current is applied through the terminals across the lamp, and usually, but not necessarily, heating current is supplied through the electrodes to heat them resistively to electron emissive temperature. In operation each electrode acts alternately as a cathode emitting electrons and being bombarded by ions, and as an anode collecting electrons. It is customary to form the electrodes as coiled coils or triple coils and to coat them with a mixture of alkaline earth metals and their oxides which imparts a low work function to the electrodes and promotes electon emission.

The coiled coil cathode has been predominant in the past because of its low cost and ease of manufacture. This type of coil works well in switch-start lamps, since the cathode damage during starting is small. Due to the wire size used and the amount of cathode coating required for long life, the coil has a resistance such that approximately volts is required to heat it to emission temperature. Continuous heating, as in the so-called rapid start systems, causes a cathode loss, of the order of 10 watts for a 40- watt lamp, which is too high for practical use.

Coils of the triple coil variety have some advantages over these coiled coils. They are very efficient in holding cathode coating material and are more resistant to ion bombardment during lamp operation. Coils of this type are used in rapid-start systems in which they are continually heated to thermal emission temperatures.

Both of these cathode designs have disadvantages. The coiled coil is simple and inexpensive to manufacture, but it is not resistant to the deleterious elfects of instant starting where the cathodes are cold, and must be brought to operating temperature by the action of the arc alone. The size of wire used is a function of lamp current and there is little design in this respect. The coiled coil does not hold the cathode coating well, with the result that it is readily sputtered off or merely knocked off during processing and handling. There is also a limit to the amount of cathode coating that can be accommodated. Larger coils capable of holding more coating are structurally weak and susceptible to damage from shock or vibration. Finally, the length of wire used must be chosen so that the voltage across the coil does not greatly exceed 12-13 v-olts, otherwise an intense mercury arc will form across the coil during switch-starting, causing severe lamp blackening.

The triple coil has a disadvantage in cost and difficulty of manufacture. It requires many turns of fine wire which is more expensive than the heavier wire used in coiled coils. Although it can cathode coating in a more eificient manner, it is harder to get the coating to penetrate the space between turns and properly fill the voids in the primary winding, resulting in cathodes which have a deficiency of coating with a resultant short lamp life. The fine primary wire is less resistant to ion bombardment than heavier wire so that it is more susceptible to damage during starting, especially if the coil is not heated. The resistance of the coil is determined largely by the size and length of the primary winding, the length of the coil and other coil parameters are restricted to insure optimum lamp operation. As mentioned above, both coil filaments are subject to harmful starting effects if the coils are not preheated. The reason for this is well understood. Until each of the cathodes is heated to a temperature at which substantial electron emission is possible, each entire cathode structure is covered with a glow discharge which subjects the relatively fragile filamentary cathode to a damaging ion bombardment. This eifect is particularly pronounced in fluorescent lamps in which the fill gas is largely neon because the lighter mass of the neon atoms offer less protection from ion bombardment than the more conventional argon or krypton fill gas. When the cathode reaches emitting temperature, the illuminating arc is established and the ion bombardment is greatly reduced.

To summarize then, the primary disadvantages of the coiled coil filament are that it does not hold the coating material well, it is not resistant to cold cathode instant starting ion bombardment, and it cannot be user in rapid start (continuous heating) systems because of heavy power losses. The triple coil is relatively expensive to manufacture, hard to coat uniformly, and also not resistant to cold cathode instant starting ion bombardment.

A third type of filament, the braided coil (C. J. Warnke, U.S. Patent No. 2,363,028), is also used in some instances. It is made by interwinding a plurality of uniform diameter wires on a braiding machine. The coil is relatively inexpensive to manufacture and is also easy to coat with a uniform layer of emissive material. However, its use in rapid start fluorescent lamps is severely restricted because its resistance cannot be adjusted to permit heating the filament properly at the low cathode voltage available in rapid start systems while using wire sizes that yield optimum lamp performance characteristics. Also, like the coils described above, the braided coil is not resistant to initial ion bombardment when employed in instant start systems (i.e. those where the filament is not preheated as in the rapid start systems).

It is the object of this invention to provide cathode filaments which are simple and inexpensive to manufacture, easily coated with a uniform layer of emissive material, capable of rapidly reaching emission temperatures so that ion bombardment damage in instant start systems is minimized, and capable of being used in rapid start systems without high power losses.

These objects are achieved by an improved design of braided coils. The filaments of this invention may be made, in whole or in part on a 16 carrier Wardwell Braider, a machine commonly used to make cloth braids, which is manufactured by the Wardwell Braiding Co., Central Falls, R.I. This machine can be readily adapted to handle fine wires, such as tungsten and molybdenum. These fine wires are interwoven around a mandrel of any desired material and configuration up to /2 inch in diameter. The resulting braid can be used directly as a lamp cathode after removal of the mandrel and cutting to length. It is coated with emissive material in the same manner as other coils.

In one embodiment of the invention, at least one of the wires has a lower resistance than the others. This design is particularly adapted for fluorescent lamps used in rapid start systems. In the braided coil described by Warnke, all the wires are of uniform diameter and resistance. This seriously restricts the application of this design to fluorescent lamps used in rapid start systems because the resistance cannot be adjusted to heat the coil properly at the low cathode voltage available in rapid start systems and at the same time use wire sizes that yield optimum lamp performance. It has now been discovered that the resistance characteristics of the braid can be regulated by using at least one wire with a lower resistance. This lower resistance wire can be either of a different material or of the same material in a heavier gauge. It will serve as the primary current carrying member. Then the remaining wires can be sized optimally to meet the discharge conditions.

A second embodiment of the invention is particularly adaptable to fluorescent lamps used in instant start systems. This design consists of a braid formed from a multiplicity of identical wires with a higher resistance wire than those used in the braid wound around the periphery of the braid. This wire can be of the same material in a finer gauge or of a different material. During lamp starting this wire overlay, being of higher resistance than the braid substructure, heats very quickly from the action of the arc discharge, causing the cathode to make the transition from glow to are in a shorter time than would be permitted by coils of other designs. Such a coil has greater resistance to the effects of ion bombardment during starting, simply because it shortens the damaging glow period when the cathode is being subjected to ion bombardment because it has not reached thermionic emission temperature.

For the purpose of illustration, the invention is shown in the accompanying drawing in which:

FIG. 1 is a side view partially in section of a fluorescent lamp incorporating a cathode structure according to FIG. 2;

FIG. 2 is an enlarged vertical section of the cathode structure of FIG. 1 with a filament illustrating one embodiment of the invention;

FIG. 3 is an enlarged view of the filament of FIGS. 1 and 2; and

FIG. 4 is a similar enlarged view of another form of filament.

The fluorescent lamp shown in FIGS. 1 and 2 comprises a glass envelope 1 having a phosphor coating 2 on its inner surface. The lamp is sealed at each end with a stern press 3 and provided with a base 4 carrying insulated terminals 6. Lead wires 8 connected to the terminals 6 support the filament 9.

According to the form of the invention shown in FIGS. 1 to 3 the filament 9 comprises a plurality of relatively small braided wires 11, over which is wound a relatively heavy wire 12. For example, a filament has been made consisting of 8 strands of 8 mg. tungsten wire braided around a 40-mil molybdenum mandrel with 178 wire cross-overs per inch. Over this a single strand of 4 mg. tungsten wire 12 at 50 turns per inch has been wound. The molybdenum mandrel is dissolved from within the braid in a known manner and the braid is cut to desired length, coated and welded at its ends to the lead wires 8.

In both this and the following form the braided filament may be coiled in various known forms other than the straight form shown prior to removal of the mandrel.

In FIG. 4 a braid of relatively fine wires 14 and one or more strands 16 of relatively larger wires are braided together, with the larger strand 16 interwoven with the smaller. For example, a braid has been made consisting of 7 strands of 4.15 mg. tungsten wire and one strand of 50.21 mg. tungsten wire braided around a 20-mil molybdenum mandrel with 178 wire cross-overs per inch. The resistance characteristics of the coil can be controlled and adjusted by the choice of wire size for the large braid wire. Wires have been used as large as 141 mg. and 5.85 times the diameter of the other braid wires.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

We claim:

1. A rapid start fluorescent lamp for use with a supply of low voltage heating current and of high voltage are current comprising an envelope containing an ionizable fill material, spaced electrode structures, and conductors connected to each said structure for connection to said supply to apply heating current through each structure and high voltage current across said structures thereby to support an alternating electron discharge are and an ion counterflow therebetween, wherein each said electrode structure comprises,

a tubular braid of a plurality of conductive strands braided about a tubular space, said strands having a resistance relatively slowly heated by said low voltage current to optimum operating temperature,

and at least one additional conductive strand of lower resistance than said braid coiled about said space coextensively with the braid and electrically connected in parallel with the braid so as to be heated relatively rapidly by said low voltage current to electron emissive state thereby to protect said braid from ion bombardment during the relatively slow heating thereof.

2. A device according to claim 1 wherein said additional strand is interwoven with said plurality of strands.

3. A device according to claim 1 wherein said additional strand circumscribes the braided plurality of strands.

4. A device according to claim 1 wherein said electrode structure is coated with an electron emissive material.

References Cited UNITED STATES PATENTS 2,052,103 8/1936 Mulder 313-2l2 2,126,155 8/1938 Van Dyck 313341 X 2,129,357 9/1938 Marden et al 313-211 2,212,827 8/1940 Stzrodt et a1. 313341 X 3,250,943 5/1966 Antonis et al 313-343 X DAVID J. GALVIN, Primary Examiner.

JAMES W. LAWRENCE, Examiner.

R. L. JUDD, Assistant Examiner, 

1. A RAPID START FLUORECENT LAMP FOR USE WITH A SUPPLY OF LOW VOLTAGE HEATING CURRENT AND OF HIGH VOLTAGE ARC CURRENT COMPRISING AN ENVELOPE CONTAINING AN IONIZABLE FILL MATERIAL, SPACED ELECTRODE STRUCTURES, AND CONDUCTORS CONNECTED TO EACH SAID STRUCTURE FOR CONNECTION TO SAID SUPPLY TO APPLY HEATING CURRENT THROUGH EACH STRUCTURE AND HIGH VOLTAGE CURRENT ACROSS AND STRUCTURES THEREBY TO SUPPORT AN ALTERNATING ELECTRON DISCHARGE ARC AND AN ION COUNTERFLOW THEREBETWEEN, WHEREIN EACH SAID ELECTRODE STRUCTURE COMPRISES, A TUBULAR BRAID OF A PLURALITY OF CONDUCTIVE STRANDS BRAIDED ABOUT A TUBULAR SPACE, SAID STRANDS HAVING A RESISTANCE RELATIVELY SLOWLY HEATED BY SAID LOW VOLTAGE CURRENT TO OPTIMUM OPERATING TEMPERATURE, AND AT LEAST ONE ADDITION CONDUCTIVE STRAND OF LOWER RESISTANCE THAN SAID BRAND COILED ABOUT SAID SPACE COEXTENSIVELY WITH THE BRAID AND ELECTRICALLY CONNECTED IN PARALLEL WITH THE BRAID SO AS TO HEATED RELATIVELY RAPIDLY BY SAID LOW VOLTAGE CURRENT TO ELECTRON EMISSIVE STATE THEREBY TO PROTECT SAID BRAIN FROM ION BOMBARDMENT DURING THE RELATIVELY SLOW HEATING THEREOF. 